There is a never ending argument among evolutionists and ethologists, do behaviors arise from innate genetic information or are they learned over the course of an individuals life? This is otherwise known as the nature versus nurture argument. Birdsong is one of these such behaviors. Similar to the human language, birds use song as a communication between individuals, a communication which is species specific. It is still unknown whether the young fledglings know how to sing the songs of their parents when they grow up. Song quality has been directly related to genetic quality, perhaps there is a gene for song composition. Although one cannot say definitively whether it is one or the other, there is much evidence for either side.
Singing is a male bird’s method of advertising his genes to receptive females in order to seduce them into mating with him. Much of the song production is dependent on the genes that a male bird has to offer. Genes code for the physiological development of a bird including the body and brain. The complex structure of the brain is integral in the production of song. All birds have a structure known as the higher vocal center (HVC). The HVC projects to the anterior forebrain pathway (AFP) and the robust nucleus of the archistriatum (RA) via Area X, which is a structure that is found only in songbirds. These pathways are integral in the production of song. If any of these structures or pathways are not produced properly, the bird may be unable to produce song. The size of the HVC is also directly correlated to the size of the repetoire of an individual bird [Ward 2001]. All of these physical characteristics are coded for by genes passed from generation to generation.
The production of song is also affected by the chemicals produced by the body. Singing is a physiological state and is effected by the interaction of multiple neuromodular mechanisms [Botas et all 2000]. Hormones such as testosterone are integral in the production of song in the male bird. During the mating season, when testosterone levels are higher in the bird’s system, the HVC is enlarged and the male bird begins to sing more frequently and for longer periods of time. He is trying to court females with his powerful singing abilities and demonstrate that his genes are superior to his surrounding competitors. Estrogen (which is often considered to be a female hormone because of its production in the ovaries) is also necessary for proper development of the male bird brain. Instead of being created in the gonads, certain male avian brain cells produce estrogen. This male-manufactured estrogen has a major effect on the development of connective tissue between the HVC and the RA (regions necessary for song production). Female manufactured estrogen will not have the same effect on the brain. When a immature female bird is injected with male-manufactured estrogen, the connective tissue in her brain begins to develop structures and pathways similar to that of a male brain. She then begins to develop a male vocal system and has the ability to produce similar songs.
A healthy motor and respiratory system is necessary for proper song production. This physical development is also coded for by genes. With a superior body, a male bird can out-sing the surrounding males and acquire the attention and desire of many females. Natural selection in birds is dependant on which male has the best song. Females are attracted to males that sing louder and longer than others. In an experiment conducted by Genter and Hulse, female starlings were caged with speakers projecting male songs. The females clearly rejected male calls that lasted for only 30 seconds when presented with calls that last a minute, they would perch closer to the minute calls. Their preference for extended singing is evident [Genter 2001]. Also, females are not attracted to the calls of other species of birds. They have the ability to distinguish the call of an species male from that of a extra-species male. this may be due to genetic information passed to the offspring
The structure of birdsong is also important when investigating its acquisition in fledgling birds. Birds sing in grouped phrases known as motifs: a motif is a fixed sequence of syllables that last at least 20ms and are separated by at least 10ms. Syllables are short series of notes emitted from the individual bird and are combined in different orders to create a language that is common among all individuals in a society. Without adult male tutors, fledglings can learn from each other as well as their mother although the songs produced by these young birds are weak imitations of songs that their older male counterparts are known to sing. The similar syllable formation leads one to believe that the ability to create syllables lies dormant in the fledgling until it has reached the appropriate age. Fledglings who have not experienced male song after approximately 120 days will develop a stereotyped song. They do not differ acoustically from adult males therefore the occurrence of song syllables are independent of tutor exposure [Helekar 1999]. The production of common tones and pitches is controlled by the body structure of each individual bird. If the bird is healthy, then it should have no problem producing the natural sounds of its species.
Helekar experimented extensively with birds that were raised by their mother alone. The female zebra finch calls but is not known to have complex song Zebra finch fledglings were allowed to interact with one another but they had no stimuli from a adult male. Although many birds will experience changes in syllable morphology including additions, deletions and splicing, it is very rare for birds to repeat syllables. Because of this low frequency, Helekar speculated that it might be genetic, resulting in a mental deficiency. Syllable repetition could be induced in zebra finches by delayed auditory feedback, creating bilateral cochlear ablations and microstimulating the HVC while the subject was singing. Helekar also found that only one pair of birds had the exact same song, all others had similar syllables but composed them completely differently. Although all of the birds eventually learned to sing, crystallization in the males occurred at a much later time that is common in the wild. There was a great variability in when the fledglings actually solidified their singing.
The AFP has been isolated as a major component of the initial learning process in birds. Lesions in the AFP during the critical period have destroyed a fledgling’s ability to learn to sing. Lesions after song has been solidified in the individual usually do not affect the bird’s singing [Perkel 2000]. Plasticity in the avian brain is also under debate. Not all birds experience a crystallization of song, for example the canary continues to learn new songs throughout its lifetime. Even though the song seems fairly static, there is some plasticity in even the zebra finches brain. They are able to change their song to adapt to their environment. As the amount of daylight changes during the year, the finch changes the daily amount of its singing to accommodate the environmental difference. The reduction in song production is thought to be due to changes associated with the circadian rhythm, neuromuscular exhaustion, and a progressive decline in motivation, therefore it is likely that the above reduction in the amount of singing during successive periods in a day is in part a behavioral reflection of some form of singing-induced neuroplasticity [Botas et al 2000].
Even though song production is possible without male tutors during the fledgling’s critical period of learning (10-50 days after hatching), the tutor-deprived males will be unsuccessful during the mating season and will not have the chance to pass on his genes. Females in bird communities are particular in their response to mating songs. Tutor learning is necessary to perfect an individual’s song quality and composition. The development of song learning is similar to that of language acquisition in humans. First there is a period of practice where the fledglings listen and attempt to emulate the sounds that are produced by the adults around them. Often it is a garbled mix of syllables but as they continue to practice, the song becomes crystallized and is set for life.
In 1964, Marler and his colleagues discovered an interesting phenomenon. There were two communities of white-crown sparrows in the local San Francisco area, one living in the Bay and the other residing in Berkeley. These two groups, although the same species, sang with drastically different dialects. In order to test the origin of these differences, he collected unhatched eggs from both communities and raised them in sound proof rooms. They were producing song within a few months but it was nothing close to the complexity of the adult males in their original communities. Marler then allowed the fledglings to listen to recording songs of male birds from both the Bay and Berkeley, the baby birds were able to reproduce the sound. He also found that it did not matter which community the fledgling originated from, he was able to learn and reproduce either dialect.
Inspired by Marler’s work, Baptista went on to explore the social aspects of song learning. He collected the same white-crown fledglings from the previous experiment and raised them in cages where they could hear and see male white-crown sparrows and strawberry finches. The fledglings could not see their siblings but they could hear them. The baby birds learned the song from their social tutor even though they could not see other white-crown sparrows. The blind recognition of the similar species call may have genetic significance but that was not expanded in Baptista’s experiments.
The development of song in birds combines many different theories regarding learning and in the end it is impossible to state that song acquisition is definitively genetic or learned during development. It is a complex and involves aspects from both schools of thought. Singing is a type of communication which must be common throughout a community. Therefore learning is essential to a male for procreation. The brain must be primed for learning. This is dependent on genetics and the environment in which an individual is raised. The similarities between acquisition of birdsong and human language are numerous. A solid explanation of birdsong will be one step closer to a solid explanation of language.
Works Cited
Baptista, L. F., and M. L. Morton.1988. Song learning in montane white-crown sparrows: From whom and when. Animal Behaviour 36:1753-1764
Gentner, T.Q., and S. H. Hulse. 2000. European starling preference and choice for variation in conspecific male song. Animal Behaviour 59:433-458
Botas, A., Espino G., Rosenfield, D. B., Helekar S. A. 2000. Reduction of female-directed song motifs induced by reated singing in laboratory-bred zebra finches. Neuroscience letters
Helekar S. A., Marsh, S. Viswanath, N. S., Rosenfield, D. B. 2000. Acoustic pattern variations in the female-directed birdsongs of a colony of laboratory-bred zebra finches. Behavioural Processes 49:99-110
Marler, P., and M. Tamura. 1964. Culturally trasmitted patterns of vocal behavior in sparrows. Science 146:1483-1486
Perkel, D. J., Farries M. A. 2000. Complementary `bottom-up’ and `top-down’ approaches to basal ganglia function. Current Opinion in Neurobiology 10:725-731
Ward, B. C, Nordeen, E. J., Nordeen, K. W. 2001. Anatomical and ontogenetic factors producing variation in HVc neuron number in zebra finches. Brain Research 904:318-326